The experiments proposed in this application are designed to gain insight into the structure and properties of the cardiac actin molecule, using immunochemical approaches. Preliminary studies have demonstrated that radioimmunoassay will distinguish tissue-specific actins. The possibility that cardiac actins possess species-specific antigenic determinants will be explored by RIA comparisons of purified cardiac actins from bovine, rabbit, and chicken hearts, using antisera against each of the three actin types. Similarities will reflect structural conservation while differences will reflect underlying variations in structure. The effect of variation in state of the cardiac actin immunogen on the tissue specificity of the resulting antibodies will be examined by immunofluorescence and immunochemistry. The possibility that the tissue specificity of cardiac actin antiserum results from antigenic determinants involving the hypervariable NH3-terminal end of the molecule will be examined by separation of antibodies against NH3-terminal determinants from those against the remainder of the molecule, by affinity chromatography, using NH3-terminal-free core fragments as the affinity ligand. Finally, the relationships between profilin, DNAase, and antibody binding to the cardiac actin molecule will be assessed as an initial probe of the surface topography of the molecule.